Why couldn't it be summer? An episode's air date doesn't necessarily have to coincide with the time setting in the show. It's been a while since I've seen the episode, but it may partly have to do with the fact that it must be summer, as waterparks, I would imagine, don't get good business in the winter, or are even open. But wasn't the park in Denver, which is a bit further from the Rockies? Not that it's at a low elevation, though.